Taking Control of Screen Coating
The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you
Direct emulsions offer screen printers mechanical endurance, solvent and water resistance, potentially high print quality, and affordability, all in one bucket. But to realize these benefits, you must have a good understanding about how to properly coat screens prior to exposure. The guidelines presented on the following pages will help you perfect your coating procedures and ensure that you get reliable, high-performance stencils every time.
Start with a clean screen
Proper reclaiming and degreasing are always first steps in coating a screen with emulsion. Anything left behind on the screen is a contaminant, including degreaser. Contaminants getting between the fabric and the emulsion create the potential for emulsion breakdown when the squeegee and floodbar shear across the screen during printing. You should examine your screenroom procedures carefully to make sure your reclaiming and degreasing efforts always result in pristine screens.
One of the most common places to find a printer adding contamination is during degreasing. If you use a power washer during degreasing, chances are high that the spray is not only hitting the screen, but also striking the walls of washout booth, where the water picks up contaminants you just removed before bouncing back onto the screen and re-depositing them. If you de-ink, reclaim, and degrease in the same washout booth, all the things you want to remove from the screen have the potential to end up back on the mesh after degreasing.
To avoid this blow-back from re-contaminating your screens, use a quality degreaser with foaming action and flood the screen as a final rinse with a high volume of water under low pressure (for example, use a garden hose without a nozzle). Rinse until the screen runs free of bubbles and water sheets from the screen smoothly without sagging or showing voids.
If water does not sheet from the fabric, degrease again. Should the water flow remain unimproved when you rinse again, you will need to apply a combination of haze remover and screen wash to remove printing residues still on the screen.
Another common cause of residue on the screen that can show up in the emulsion coating is water and other chemicals that hide in the profile of retensionable frames. Generally this problem is at its worst when the screen is dried in the upright position. Water and contaminants trapped in the profile run down the screen after degreasing. The contamination dries on the screen and later interferes with a good bond between the fabric and the emulsion. For this reason, always give retensionable frames a good wipe after degreasing and store them parallel to the floor when drying.
Abrading mesh scratches the surface of the threads, which aids in stencil adhesion but is usually only required when using indirect stencils. One reason not to abrade mesh is that you may decrease the ink-transfer efficiency of the final printing screen. Because the surface area of the thread is effectively larger, the ink clings to the threads for the same reasons capillary film clings better. This may result in less ink from the mesh transferring to the substrate. Another reason to avoid abrading the mesh is that it will likely cause more ghost haze and stains to remain in the mesh after reclaiming, also due to the increased surface area.
The stencil’s main function is to allow for accurate reproduction of the image from the film positive (or from the image rendered by a computer-to-screen imaging system). The quality of the stencil pertains to how effectively it bridges the mesh and the amount of resolution and edge definition it provides after exposure. These abilities vary dramatically for any given emulsion because of several factors related to coating and exposure, as well as the chemical characteristics of the emulsion.
One of the main functions of the stencil is to create a gasket with the material being printed. Among the reasons why capillary stencil films are popular is because their uniform surface leads to a good gasket right off the roll. With proper control during coating, direct emulsions also can be used to create similarly smooth stencils.
The degree of roughness of the stencil surface is measured as an Rz value. Basically, the lower the Rz value, the smoother the surface and, therefore, the better the seal that occurs between the substrate and stencil during printing. By adjusting the coating technique, users of direct emulsions can fine-tune the Rz value of their stencils for any particular substrate. Such adjustments are necessary to achieve accurate and consistent prints on all materials.
The key to proper gasketing between stencil and substrate is for the emulsion to provide a shoulder that maintains space between the substrate and the mesh in open areas of the stencil. Because ink cannot travel through mesh threads, having some emulsion thickness over (on top of) the mesh plane helps to define the edges of the image (Figure 1). (Figure Reference: EOM levels of 10-20% are generally recommended for screen printing (no more than 10% for process-color printing, particularly with UV inks. The extra emulsion thickness provides a shoulder at the mesh openings that keeps the mesh and substrate from making contact, which allows ink to transfer from the entire open area, leading to good image definition and dot shape and size.) Without this emulsion over mesh (EOM), the risk of sawtoothing and dot loss increases, and fine detail printing becomes virtually impossible.
Fine detail can be taken to mean any artwork in which the finest elements are equivalent to the width of two threads and two mesh openings of the particular mesh fabric being used. You can test this concept by laying your film positive right on the screen and looking through a 50x microscope to see how many mesh openings and threads your finest design elements span. If your smallest dots or lines are not much larger than two threads and two mesh openings, you have fine detail. With such details, if you do not have enough EOM, the threads will create a dam around open areas in the same way that a stencil does. In halftone printing, such low EOM leads to dot loss as the highlight dot is cut into pieces by the threads (Figure 2). (Figure Reference: With insufficient EOM, the mesh in open areas of the screen can make direct contact with the substrate during printing. As a result, the knuckles of the mesh serve as dams that prevent ink from spreading through the entire open area. The result is dot loss and poor dot reproduction.)
Stencils with low EOM also likely suffer from high Rz values. The incomplete gasket caused by high Rz allows the ink to push beyond the image edge and leak into the valleys of the stencil. This results in dot gain. At the same time, the low EOM leads to dot loss and sawtoothing. High Rz and low EOM are both the result of poor coating techniques. However, these conditions can be eliminated by observing the screen-coating pointers we’ll consider in the next section.
All stencils must have some EOM for proper printing. But, a stencil can be too thick as well. For many printing inks, stencils perform well with an EOM that is between 10-20% of the mesh thickness. For finest detail printing, including four-color process, the EOM should not exceed 10% of the mesh thickness. If the stencil becomes thicker, problems such as ink piling, skipping, and skewing can occur when printing with UV ink. Moreover, highlights may not open up, and ink transfer becomes difficult. For excellent printing on most substrates, the stencil will also need a low Rz value (approx. 6 microns if possible).
The coating process
The process of screen coating includes a base-coating procedure, a drying stage, and, for some applications, an additional face-coating procedure.
Base coating Creating the base of emulsion on the screen is typically referred to as wet-on-wet or base coating (Figure 3). (Figure Reference: Base coating fills a clean screen mesh with emulsion and is responsible for building up stencil thickness. With a round-edged coating trough, apply coats of emulsion to the substrate side of the screen until the emulsion flows completely to the squeegee side. Then turn the screen around and coat as many passes on the squeegee side as it takes to build up the required thickness on the substrate side. Finally, dry the screen with the substrate side down.) Each successive pass with the coating trough adds more emulsion to the wet emulsion already on the mesh. The important thing to be aware of is that base coating accumulates emulsion on the side of the screen opposite the coating trough.
For most mesh counts you will do all your base coating with a round-edge coater. Always start coating from the substrate side of the screen, and coat as many times from the substrate side as is necessary to cause the emulsion to visibly gloss on the squeegee side. The purpose is to push out all the air bubbles that may be trapped in the mesh openings. If left in the mesh, this air can cause print-quality defects or pinholing on press.
Note that finer mesh counts may require more coats than coarser mesh counts. Mesh regulates emulsion flow in the same way that it meters, or regulates, the amount of ink going to the substrate. As the percent open area of a mesh decreases, it restricts the flow of emulsion. For example, a 380-thread/in. mesh with a 34-micron thread diameter may require three coats from the substrate side followed by two coats from the squeegee side to build a 3-micron EOM. In contrast, a 380-thread/in. mesh with 27-micron thread diameter might have a 5-micron EOM after only one coat on the substrate side and one coat on the squeegee side. Although the mesh count is the same, the mesh with the 27-micron thread diameter has much more open area and less resistance to flow because the emulsion doesn’t have to push against so much thread.
Watching for the glass-like glossy finish to occur on the squeegee side, while coating from the substrate side, will indicate the first moment where the emulsion has filled the mesh completely. Each pass with the coater after this point causes the EOM to build. This change in thickness is fairly predictable.
To build up your EOM on the substrate side of the screen, simply turn the screen around and begin coating from the squeegee side. Each pass with the coater will continue to build wet emulsion thickness proud of (above) the mesh. How much the EOM builds with each pass depends in part on the coater design.
Drying After base coating, the screen is dried with the substrate side down and the squeegee side up. This allows gravity to pull emulsion to the substrate side of the screen where you want it. Not only will you build EOM, but you will also lower the Rz values.
Face coating The base-coating process allows you to lower Rz value at the same time that you’re building EOM, and the resulting screens are often perfectly useful for many applications. Unfortunately, printers who rely solely on wet-on-wet coating may end up with a stencil that is too thick before they reach an acceptable Rz value. The solution to this problem is face coating rather than trying to lower Rz through base coating.
Face coating is applied for the purpose of leveling the emulsion coating on the screen (Figure 4) (Figure Reference: Fine-detail printing often requires stencils that are both thin and have a low Rz to provide good gasketing during printing. To achieve such stencils, screens are base coated with only enough emulsion to completely fill the mesh openings and leave a slight EOM. After the screens are dried, a sharp-edge coating trough is used to coat the face of the screen, filling in any depressions between mesh knuckles so that the surface is uniform. The back (squeegee side) of the screen may also receive an additional thin coating of emulsion.) This reduces the Rz value and helps the stencil form a gasket during printing. Face coating is done after the base coating is dry. When face coating is performed with a sharp edged coater, it has the advantage of lowering the Rz with little change in stencil thickness (typically, less that a micron per coat).
Face coats are usually applied from the substrate side of the screen, but face coating from the squeegee side in addition to the substrate side has shown to improve print quality. In both cases, the Rz of the stencil is lowered because the coating trough plows over the high spots (mesh knuckles) and leaves emulsion only in the low spots (mesh openings). By combining base coating with face coating techniques, the printer has the opportunity to adjust the stencil for any substrate and for any type of printing.
After face coating, screens must be dried a final time. However, unlike after base coating, when screens are dried substrate-side down, face-coated screens are dried squeegee-side down.
Coating trough design
The shape of the lip of the coating trough has a great influence on the amount of emulsion that is deposited with each pass of the coater. As mentioned previously, it’s generally necessary to use a coating trough with a round edge (approximately 1⁄8 in. diameter) for the wet-on-wet base coats to have much influence on building EOM.
For example, consider the common emulsion buildup one will generally get with a round-edge coating trough on a 380-thread/in. mesh with 34-micron thread diameter. With two coats on the substrate side and one of the squeegee side, you can expect a 1-micron EOM. With two coats on both sides, the EOM would be 3 microns. And with two coats on the substrate side and three on the squeegee side, the resulting EOM is 6 microns.
Now let’s compare those results with the effects of coating the same mesh with a sharp-edge coating trough. The 2+1 coating regimen in this case would provide a 0-micron EOM, the 2+2 coating would give a 1 micron EOM, and the 2+3 approach would result in an EOM of 2 microns. For most applications, the number of coating passes that would be required to build a sufficient EOM make the use of a sharp-edge trough impractical.
A final guideline for coating trough use is to make sure there always is sufficient emulsion in the coater. The fill level is the driving force for pushing emulsion through the mesh. If this level varies from screen to screen, so will your results. When coating large screens, fill the coater each time you begin a new screen to keep the coating thickness consistent.
Build in consistency
Design your coating area so that your staff can more easily achieve consistent coating results. Your screens should all be at the same angle each time they are coated. Any change in angle will change the amount of emulsion that passes through the mesh. A coating stand works best to control the angle. All screens must also be coated at a consistent speed along their length. And the same coating speed should be used from screen to screen. Any increases or decreases in speed result in differences in the amount of emulsion deposited on the screen. Using an automatic coating machine is the best way to ensure coating consistency.
Let the emulsion flow
Effective screen coating requires you to carefully control EOM and Rz and understand what combination of coating techniques and tools is the best match for the mesh types you use and the jobs you print. As in all other areas of the screen printing process, establishing and adhering to standards in your screen-coating procedures will lead to improved quality, repeatability, greater efficiency, and lower costs.